The invention relates to an optical system, in particular a projection system for microlithography, in particular exposure with a slit-shaped image field or which is not rotationally symmetrical, which has an optical element, in particular, a lens or a mirror which is arranged in a mount, and actuators which engage a portion of the mount and/or the optical element.
An optical system of the kind mentioned hereinabove is described in EP 0 678 768 A2. Step and scan processes are used therein, whereby only a narrow, slit-shaped strip of a mask is transferred to a wafer. In order to expose the whole field, a reticle is used, and the wafer is displaced sideways (scanning).
However, it is disadvantageous in this case that a rotationally asymmetrical illumination impression arises, above all at the lens near the wafer, due to this slit geometry. This means that the temperature distribution on the lens arising due to the unavoidable lens heating is likewise rotationally asymmetrical and therefore leads, due to the linear dependence of refractive index on temperature and thermal expansion, to image errors, e.g., astigmatism, on the optical axis.
In 193 nm lithography, the 193 nm light passing through the quartz glass lenses leads to a volume decrease of the quartz glass, increasing monotonically with NI2. Here N is the number of laser pulses and I is the pulse dose. Furthermore, an increase of refractive index results. Since the increase of refractive index overcompensates for the decrease of optical path length due to the shrinkage, the consequence of this effect termed compaction is a disturbance of the wavefront. This leads, just like a lens heating, to image errors such as astigmatism on the optical axis.
In contrast to a compensation of the lens heating, there is no passive compensation for the compaction effect. Here the change of the wavefront had to be compensated actively, by changing a lens element. Since it is not possible to use an active mirror in a refractive design (the cost of introducing an additional mirror for image error compensation in general excludes this), one or more lenses have to be used as xe2x80x9cadjusting membersxe2x80x9d. In order to correct astigmatism on the axis, movements along the optical axis, and also decentering, are excluded. Hence all translational degrees of freedom are unavailable as possibilities for a correction.
In EP 0 678 768 it has now been proposed to use a lens as the xe2x80x9cadjusting memberxe2x80x9d, in order to correct the image errors produced by a non-uniform heating of the lens. For this purpose, it is provided according to FIG. 11 to allow forces acting in the radial direction to act on the lens. However, only an asymmetrical change of thickness is produced by the pressure forces thus produced on the lens.
In EP 0 660 169 A1, a projection exposure device for microlithography is described, in which the objectives are provided with correction elements. For this purpose, among other things, a lens pair is provided, which is rotatable around the optical axis. The refractive power is thereby altered by the shape of the lens by the superposition of a cylindrical meniscus shape over a spherical lens.
The present invention has as its object to provide an optical system of the kind stated at the beginning, in which the image errors unavoidably arising due to the non-uniform temperature distribution can be corrected or minimized with simple means.
According to the invention, this object is attained by the features stated in the characterizing portion of claim 1.
In contrast to the prior art, not only are pressure forces produced which result solely in an asymmetrical thickness change, but also a bending of the optical element, for example, a lens, is brought about by means of the thrust forces or torsion which re produced, and is chosen so that the unavoidably arising image errors are compensated to the greatest possible extent. With the actuators according to the invention, an optical element, such as for example a lens, can be controllably deformed by a few 100 nm up to xcexcm. In this manner, for example, a compensation of astigmatism r2 and r4 can be attained.
In the process according to the invention, the desired temperature distributions can be quickly and reliably attained with simple means. This is in particular the case when only certain image errors, for example, image errors of low order, are to be corrected.
A further very important advantage of the invention is that if necessary xe2x80x9covercompensationsxe2x80x9d and the additional compensation of production errors are possible. Instead of a symmetrizing of several individual lenses, as is the case in the prior art, it is also possible to xe2x80x9covercompensatexe2x80x9d individual lenses, i.e. to intentionally make the temperature distribution or deformation asymmetrical xe2x80x9cin another directionxe2x80x9d. In this manner, the overall result is a compensation of the whole objective or of the illumination device.
As regards the compensation of production errors, there are two variants, namely a simultaneous compensation of inadvertent production errors and an intentional building in of a fixed deflection, in order to halve the required amount of correction.
With the process according to the invention, a simultaneous compensation is possible of lens heating and of the compaction effect of the optical element.
The optical system according to the invention can be used with particular advantage in semiconductor lithography, since the image errors arising due to the progressive scaling down of the structures to be imaged likewise have to be minimized.
With the actuators according to the invention, an astigmatism can be controllably produced for the compensation of the thermal astigmatism and the effects due to compaction in the optical element, e.g., a lens.
It is also of advantage that it is furthermore possible, in dependence on the arrangement and number of actuators, to produce other deformations of the optical element.